76 ORGANIZATION OF HIGHER PLANTS 



Exercise XV 



THE ASCENT OF SAP 



The cells in the root have a higher osmotic 

 pressure than the salt solution in the soil. This 

 so-called root pressure draws water in from the 

 soil and pushes it upward through the vessels of 

 the xylem. A second source of osmotic pressure 

 originates in the leaves. Here water is con- 

 tinuously evaporated from mesophyll cells, and 

 water vapor finds its way to the exterior through 

 the small openings in the leaves called stomata. 

 This loss of water from the leaves (transpiration), 

 since it concentrates continuously the contents of 

 their cells, creates an osmotic pressure that tends 

 to draw water into them from adjoining cells, 

 and eventually from the sap-filled conducting 

 vessels of the xylem. 



As already noted, if this removal of water 

 from the vessels of the xylem acted by creating a 

 vacuum at the top of the sap column, that could 

 at most develop a pressure of 1 atmosphere and 

 could raise sap at most 34 feet. It is now realized, 

 however, that owing to the great cohesion of 

 water (why has it so great a cohesion?), a con- 

 tinuous column of water can support a tension 

 of at least 20 to 30 atmospheres and perhaps 

 much more before breaking. Added to this, the 

 cellulose walls of the xylem and its conducting 

 vessels not only imbibe much water (up to 30 

 to 40% of the plant's dry weight) but also bind 

 the water of the sap columns by powerful elec- 

 trostatic forces and hydrogen bonding to the 

 — OH groups of cellulose. Very long columns of 

 sap can be lifted by this combination of forces: 

 transpiration of water pulling from the top, the 

 internal cohesion of the sap column, soaking 

 up of water through imbibition by the cell walls, 

 and the adhesion of the sap columns to the walls 

 of the vessels. 



This entire view of the process is spoken of as 

 the transpiration-cohesion-tension theory of sap 

 rise. It appears principally to account for the rise 

 of sap in trees in full leaf, with root pressure as a 

 secondary force. Transpiration pulls and root 

 pressure pushes the sap upward. In the early 

 spring, of course, before the leaves appear, there 

 is little if any transpiration, and the sap must 

 ascend mainly by root pressure. 



EXPERIMENTS 

 Plasmolysis 



Sucrose, though a small molecule, enters cells 

 only very slowly. If a plant cell is placed in a 

 sucrose solution whose concentration is greater 

 than that of the total dissolved contents of the 

 cell (i.e., a hypertonic solution), water leaves the 

 cell. When enough water has left, the protoplasm 

 of the cell within its plasma membrane contracts 

 away from the cell wall. This process is called 

 plasmolysis. The concentration of sucrose at 

 which plasmolysis just becomes detectable is 

 equivalent to the osmotic concentration of the 

 cell contents. 



We shall determine in this way the osmotic 

 concentration of epidermal cells of the red onion. 

 With scalpel and tweezers remove strips of 

 epidermis. The color of these cells is due to a 

 red, water-soluble anthocyanin pigment, and 

 will help you to detect the first withdrawal of the 

 cytoplasm from the cell wall. 



Place strips of epidermis in a graded series of 

 sucrose solutions ranging in concentration from 

 0.1 to 0.6 M. Leave them for 30 to 60 minutes, 

 and then determine the degree of plasmolysis by 

 observing them under low power in the com- 

 pound microscope. From your observations 

 estimate the approximate osmotic concentration 

 of the cells. What osmotic pressure (in atmos- 

 pheres) should they develop when placed in 

 water? How high could this osmotic pressure 

 raise a column of water? 



Transpiration 



Water absorbed by the roots travels through 

 the vessels of the xylem, which form a con- 

 tinuous conducting system from the young roots 

 to the mesophyll tissue of the leaves. Most of 

 the water absorbed by a plant in leaf is lost by 

 evaporation from the surfaces of the mesophyll 

 cells. The water vapor finds its way through 

 intercellular spaces in the mesophyll to the ex- 

 ternal air via the stomatal openings. 



A geranium plant is available for each two 

 students. The device for measuring transpira- 



